The invention relates to charge pump device and a display driver with a charge pump device. Further it relates to a display module with a display driver using a charge pump device and a telecom terminal having such a display module.
Charge pump devices or voltage multipliers are used in devices, where a higher voltage than the supply voltages is necessary. These charge pump devices will be used especially in LCD modules and in flash memories, because the highest voltage required is larger than the voltage supply available. These parts are nowadays produced in large quantities and the current efficiency is a major issue. They are intended to be used in battery-operated applications, so low current consumption and recently low voltage become more and more important.
The LCD modules find a large application in cellular phones and other hand-held devices like organizers, laptops, PDAs, etc. In the phones, the supply voltage available for analog blocks is 2.8V and a LCD graphic display can be driven with voltages of 6 to 16V. The voltage needs to be multiplied to at least 3 times of the supply voltage. Because a display should be able to operate within a large range of the supply voltage, the multiplication factor should vary with the application.
A charge pump is composed by a cascade of several stages, whereby a stage contains at least a switch or a diode, a charge storing element, mostly realized as a capacitor and one driver. The driver commands the charge storing elements and is operated by periodical signals or phases. A charge stored in the first stage will be forwarded to the next stage, where it will be added to the charge of this stage, so a higher voltage will be generated, which is provided to the devices i.e. an LCD Module in a mobile phone.
There are two kinds of charge pumps:
with on-chip capacitors or
with external capacitors.
The on-chip capacitors driver is the simplest solution for Chip-On-Glass realizations and can offer a current efficiency of about 95%. The costs for the one-chip solution are lower. The charge pump with external capacitors is more efficient, but less suited to the Chip-On-Glass solutions because it is more expensive.
In order to change the multiplication factor of a charge pump to adapt the solution to different applications it would be possible to use not all stages of the charge pump to generate output voltages below the maximal possible voltages, where all stages of the charge pump are used. For example, one of the characteristics of the liquid crystal of the display is that it must be driven with a higher voltage at low temperatures, while the battery is only able to supply a low voltage. In this case, selecting a higher multiplication factor than at room temperature could improve the current consumption of the display module.
In the present case, even with not so low supply voltage Vdd, when not all the stages are used, it happens that one of the middle stages is used as the first switching stage, bringing a current efficiency loss.
So the charge pump is functional but it consumes more power than the optimum, because of the back current. To minimize the losses due to the back current, which is flowing from one stage to the stage before that stage towards the input of the charge pump device, while the switch in the looked stage is not yet completely open, the switches have been improved.
To solve the problem with the back current the switches of the first stage and the last stage of the charge pump are different from the middle switches.
In known applications, the charge pump has been designed including a mask change, with which the first used stage was transformed to increase efficiency, but forbidding further software changes of the multiplication factor. This solution is quite restrictive.
The EP 03190634 describes a voltage multiplier circuit comprising a serie of rectifier elements, which are alternately rendered conductive by alternatively applying complementary clock signals to capacitances which are connected to junction points of pairs of neighboring elements.
In a programmable charge pump, the multiplication factor is determined by how many stages are switching. The stages closer to the input, beginning with the first stage, are turned conductive to bring the supply voltage at the input of the first switching stage. The intermediate or middle stages are all of the type in FIG. 2, with bootstrap capacitor to increase the level of the command gate.
In a programmable charge pump, the first switching stage can though be any one of the stage.
By using that kind of stages or elements as a first stage of a voltage multiplier or a charge pump the input is constant (Vdd) and can not be switched as it would be a middle stage. In this case, the switch can not be turned ON if the supply voltage is lower than 2|Vtp| (Vtp is the threshold voltage of the PMOS).
As the supply voltage Vdd becomes lower and lower, this problem arose. The switches used in nowadays charge pumps are built with PMOS transistors as switches of the stage. To improve the behavior of those switches, either bootstrap capacitors or level shifters are used to drive the gate of a PMOS switch. Bootstrap capacitor technique for all the middle and the last stage is used and recognized as a very efficient solution. But the switches can operate correctly only for a supply voltage Vdd greater than 2|Vtp|, where Vtp is the threshold voltage of a PMOS transistor. The limitation is in case a switch with bootstrap capacitor which is used as first stage and supplied between Vdd and Vss permanently, whereby Vss is ground of the switch, assumed as 0V, but it can vary because of parasitic resistances. For lower supply voltage, the charge pump will not work. A |Vtp| can be as high as 1.3V, varying with the process parameters and the temperature. Therefore, any Vdd lower than 2.6V will cause a problem.
So it is an object of the invention to provide a device with a charge pump with a freely adjustable multiplication factor and minimized power losses.
The object will be solved with a charge pump device containing at least two stages, whereby a stage comprises a switch and a charge device which are arranged to generate a certain voltage higher than the supply voltage, whereby the stages are arranged in serie and a required multiplication factor of the charge pump is adjustable by activating/deactivating a certain number of stages and the switches of each stage are arranged in the same way.
The invention deals with that limitation of the low supply voltage and also constitutes a solution to efficiency loss when the multiplication factor is programmed lower than the maximum. The improvement concerns the switches with bootstrap capacitors. The new charge pump is functional beginning with Vdd=|Vtp| and even programming a different multiplication factor than the number of stages N is not reducing the current efficiency.
An embodiment of a charge pump device according to that invention is characterized in that for a adjusting a multiplication factor smaller than the maximal possible factor the stages beginning from an input of the charge pump device will be deactivated. By this the possibility for using this charge pump for different application is assured, whereby due to the switching off of not needed stages no power loss appears.
So for applications where not the maximal multiplication factor is needed, the not used stages can be switched off without any the loss of power.
To achieve this the charge pump is composed of a cascade of switches, drivers and charge devices, realized as stage capacitors. Each switch SW of a stage comprises a switch MP1 which is arranged between an input IN and an output OUT of the stage (S) of the charge pump device, further two transistors MP2 and MP3 for controlling the isolated bulk of the switch MP1 and a fourth transistor MP4 to charge a boot capacitor (CB), whereby the boot capacitor (CB) is arranged for storing the charge to drive the gate of the switch MP1 further comprises a gate switch control unit GSU, whereby the gate switch control unit GSU is arranged to switch the gate of the switch MP1.
PMOS transistors have isolated bulk because they are built in a N-well area. By biasing this N-well area always to the highest potential the junction is reverse biased and isolates the PMOS transistor electrically from the substrate. The role of MP2 and MP3 is to determine which is the highest potential between the input and the output. All of the PMOS transistors of one switch are built in the same N-well area. In an embodiment of that invention the switch MP1 is preferably realized as isolated bulk transistor.
In an embodiment of that invention the charge pump device containing a level generation unit (LGU) for providing control signals for the gate switch control unit (GSU), whereby the gate switch control unit is foreseen to connect or disconnect the gate of the switch MP1 transistor from the CB capacitor.
In an preferred embodiment of the invention a charge pump device has a gate switch control unit, which is arranged to provide control signals to switch MP1 of the stage in case of voltages below Vdd.
By using the gate switch control unit GSU and the level generation unit LGU it is achieved to disconnect the bootstrap capacitor Cb from the gate of MP1, when MP1 needs to be turned ON and driving directly with Vdd. When it needs to turn MP1 OFF, MN1 become OFF and MP5 is ON. The other two transistors MN2 and MP6 are to command correctly the gate of MP5. In the inventive solution the two transistors MN1 and MP5 located in the GSU are not conducting in the same time. MN1 is active when the switch MP1 has to be turned conductive: we apply 0V (alias less than switchb greater than ) to the gate of MP1 instead of the bottom plate of the bootstrap capacitor Cb that can be |Vtp| in that moment and then it will be disconnected by the MP5, not conducting because its gate is to the selected highest potential.
So with that new architecture it is possible to realize the charge pump with switches having bootstrap capacitors in all stages and additionally the charge pump will be working at Vdd=|Vtp|.
Through it all stages can be build or realized in the same way, which leads to the results that by programming a multiplication factor lower than the maximal one, one of the middle stages of the charge pump can be used as first stage by avoiding the loss of power.
Further by using that kind of stages of a charge pump device it is possible to reprogram a multiplication factor after using a certain first multiplication factor.
The object will also be solved by a display driver for providing display information and voltages to a display unit with a charge pump device containing at least two stages, whereby a stage comprises a switch and a charge device which are arranged to generate a voltage higher than the supply voltage, whereby the stages are arranged in serie and all stages are realized in the same way and a required multiplication factor is adjustable by activating/deactivating a certain number of stages.
Further the object will be solved by an display module having display unit and a display driver with a charge pump device, containing at least two stages, whereby a stage comprises a switch and a charge device which are arranged to generate a voltage higher than the supply voltage, whereby the stages are arranged in serie and all stages are realized in the same way and a required multiplication factor is adjustable by activating/deactivating a certain number of stages.
The object will also be solved by an Telecom terminal having a display module (DM), a display unit (DU) and a display driver (DD) with a charge pump (CP) device, containing at least two stages (S), whereby a stage comprises a switch (SWn) and a charge device (CSn) which are arranged to generate a voltage higher than the supply voltage (Vdd), whereby the stages (S) are arranged in serie and all stages (S) are realized in the same way and a required multiplication factor (MF) is adjustable by activating/deactivating a certain number of stages (S).
In battery operated telecom terminals it is very important to have an LCD module with a minimized power consumption, due to a higher Standby or operating time.